The broad, long-term objective of this application is to provide a better understanding of the basic cellular immune mechanisms by which protected individuals and animals control Plasmodium development in the liver. Despite the severe impact of malaria on millions of people, the basic immunobiology of this parasite in the liver, the initial site of replication in the mammalian host, is still poorly understood. The ability of malaria-specific effector T cells to successfully target the parasites and eliminate them from the liver has been well established using the attenuated malaria sporozoite vaccine model. This whole organism approach represents the "gold standard" for a malaria vaccine, since immunization of rodents, monkeys and humans with radiation-attenuated sporozoites, delivered by intravenous injection or the bite of malaria-infected mosquitoes, was shown to prevent blood infection. Understanding which of the possible cellular mechanisms of cytotoxicity provides the highest level of protection may help refine vaccination strategies to induce such responses and improve the chance that more practical vaccines can be found. For example, multivalent subunit vaccines offer numerous advantages including greater safety due to minimized deleterious responses against non-protective determinants, lower production costs, and ease of storage. Ideally, such a vaccine would prevent the 2-3 million malaria deaths worldwide, an estimated 90% of which occur in young children in Africa. The Specific Aim is to exploit the recent advances in microscopic imaging to determine how Plasmodium liver stage development and merozoite release can be interrupted in an immune host. Novel imaging techniques and instrumentation, transgenic parasites and mice, highly specific fluorogenic reporter substrates and molecular probes will be used to elucidate, which liver cells are able to present malaria antigen to effector T cells and which cellular effector mechanisms eliminate the parasites from the liver. Direct visualization and molecular analysis of target cell killing, the pinnacle of T cell immunity, will greatly advance our understanding of the adaptive cell-mediated immune response against malaria liver stages.